This invention relates to polycarbonate polymers and more particularly to pigmented polycarbonates which are used to form moulded articles. 2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent physical properties. These thermoplastic polymers are particularly suited for the manufacture of moulded products where impact strength, rigidity, toughness and excellent electrical properties are required.
In many polycarbonate uses, an opaque moulding is required to provide aesthetic qualities to the moulded article and/or to provide protection from light to materials to be stored within containers moulded from polycarbonates. Because of high viscosity of the polycarbonate resins and pigmented polycarbonate resins a high degree of force is necessary to mould polycarbonate articles. The rheology of pigmented polycarbonates causes problems in moulding articles which have radical contours and fine interstices in the moulds used to form these articles which restrict the flow path of the thermoplastic resin.
In many instances pigmented polycarbonate articles, which have been made in moulds having such radical contours and fine interstices, exhibit a striation in surface appearance. This striation is not a discernible irregularity in the surface smoothness of the article, but an irregularity in color characterized by definite tonal differences having discrete boundaries on the polycarbonate surface. The striation substantially detracts from the appearance of the moulded polycarbonate article making pigmented polycarbonates somewhat undesirable for moulded articles having radical contours and fine interstices.
The exact mechanism which causes the striation is not known, but it is theorized to be caused by a melt fracture in the moulding which does not reknit.
In accordance with the present invention, a pigmented polycarbonate is provided which can be moulded into articles from moulds having radical contours and fine interstices with constricted flow paths, while producing a moulded article with excellent surface properties.